1. Field of the Invention
The present invention relates to optoelectronic devices. More particularly, the present invention relates to imaging arrays based on charged coupled devices (CCD) formed from semiconductors such as gallium arsenide (GaAs) which are capable of generating electrical signals in response to light. The invention is particularly applicable to the imaging and the telecommunications arts, although it is not limited thereto.
2. State of the Art
In the imaging arts, next generation imaging systems must operate at very high frequencies and have a high resistance to radiation flux such that the imager is said to be “radiation hardened.” State-of-the-art imagers are constructed as silicon integrated circuits in the form of CCDs or active pixel arrays. In the CCD, a linear array of pixels is clocked sequentially to a common output amplifier. In the active pixel array, the array is x-y addressable and each pixel is output to its own dedicated amplifier (the array is output on a row by row or column by column basis).
Silicon technology is limited by the presence of the silicon oxide in both the active and passive regions of the integrated circuit in a number of ways. A main limitation is the sensitivity of the oxide to radiation flux. The radiation creates traps and other charged defects in the insulator which alter the internal voltage thresholds in both active and passive regions within the integrated circuit. After a certain cumulative exposure level, these threshold changes render the circuit inoperable. The gate oxide creates limitations in other ways as well. The silicon CCD couples one pixel to the other via overlapping gates. Each overlapping gate creates a small region of thicker oxide between pixels which inhibits charge transfer and therefore sets a speed limitation upon the CCD. These oxide barriers are fundamental to the silicon CCD and constitute a transfer speed limitation. Some approaches have been employed to eliminate these effects such as the virtual phase CCD. However, these structures are then faced with barriers created by implant misalignment and a lack of well capacity. In any event the transfer speed in the silicon CCD rarely exceeds a few MHz.
A further limitation of the silicon CCD is its spectral sensitivity. The silicon CCD absorbs radiation across its energy gap and therefore is insensitive to radiation with a wavelength longer than about 1 um. It is also insensitive to ultraviolet (UV) radiation.
As disclosed in parent application U.S. Ser. No. 09/556,285, III-V device structures based upon GaAs substrates have the potential to overcome the above limitations. In particular, the GaAs CCD has the potential to absorb electromagnetic energy within a quantum well between the various subbands. This provides the GaAs device with unique capabilities of intersubband absorption and sensitivity in the mid wavelength infrared, long wavelength infrared and very long wavelength infrared regions. The GaAs device structures that currently perform the intersubband detector functions are the QWIP (quantum well infrared photodetector) devices. Two significant limitations of the QWIP as currently implemented are the existence of a significant level of dark current that necessitates cooling of the device to 77° K., and the fact that the device is not compatible with GaAs integrated circuits. When originally demonstrated, the QWIP was considered advantageous because of its potential compatibility with GaAs integrated circuits. However, this compatibility has never been established and so present technology combines the GaAs QWIP wafer in a hybrid fashion with a Si read-out integrated circuit.
There have been several efforts to build CCD shift registers using the basic transistor structures of the MESFET (metal semiconductor field effect transistor) and HEMT (high electron mobility transistor) devices. See Song et al., “A Resistive-Gate Al0.3Ga0.7As/GaAs 2DEG CCD with High Charge-Transfer Efficient at 1 GHz,” IEEE Transactions on Electron Devices, Vol. 38, No. 4, April 1991, pgs. 930-932; Ula et. al., “Simulation, Design and Fabrication of Thin-Film Resistive-Gate GaAs Charge Coupled Devices,” Electron Devices Meeting, 1990, pgs. 271-274; Bakker et al., “A Tacking CCD: a New CCD Concept,” IEEE Transactions on Electron Devices, Vol. 38, No. 5, May 1991, pgs. 1193-1200; Davidson et al., “GaAs charge-coupled devices”, Can. J. Physics, Vol. 67, 1989, pgs. 225-231; Song et al., “Characterization of Evaporated Cr—SiO cermet films resistive-gate CCD applications,” IEEE Transactions on Electron Devices, Vol. 36, No. 9, September 1989, pgs. 1575-1597; LeNoble et al., “A Two-Phase GaAs Cermet Gate Charge-Coupled Device,” IEEE Transactions on Electron Devices, Vol. 37, No. 8, August 1990, pgs. 1796-1799; Beggs et al., “Optical charge injection into a gallium arsenide acoustic charge transport device,” Journal of Applied Physics, Volume 63, Issue 7, 1988, pgs. 2425-2430; Ablassmeier et al., “Three-phase GaAs Schottky-barrier CCD Operated up to 100-MHz Clock Frequency,” IEEE Transactions on Electron Devices, Vol. 27, No. 6, June 1980, pgs. 1181-1183; LeNobel et al., “Uniphase operation of a GaAs resistive gate charge-coupled device,” Can. J. Physics, Vol. 70, 1992, pgs. 1143-1147; LeNobel et al., “Two-Phase GaAs cermet-gate charge-coupled devices,” Can. J. Physics, Vol. 69, 1991, pgs. 224-227; Ula et al., “Optimization of thin-film resistive-gate and capacitive-gate GAAs charge-coupled devices,” IEEE Transactions on Electron Devices, Vol. 39, No. 5, May 1992, pgs. 1032-1040; and LeNoble et al. “The Surface Potential Variation in the Interelectrode Gaps of GaAs Cermet-gate Charge-Coupled Devices,” Solid-State Electronics, Vol. 33, No. 7, 1990, pgs. 851-857. These technologies have always been plagued by the problem of low transfer efficiencies between pixels in the array. The proposed solutions utilize a resistive coupling between pixels, which would provide drift aided transfer. The problem has been that no viable technique to implement resistive coupling has been found. The use of deposited resistive layers was attempted but the resistive control problems discouraged further investigations.
Parent application U.S. Ser. No. 09/556,285 overcomes many of these problems by providing a CCD having an epitaxial growth structure which utilizes a modulation doped quantum well interface to create an inversion channel for the storage of charge packets. The charge transfer is facilitated by the unique features of the epitaxial growth which include two delta-doped sheets of p-type doping. One p type sheet is very close to the inversion channel and enables a resistive coupling between adjacent pixels through a very thin sheet of highly doped material. The resistive coupling enables a high field and optimized drift velocity between pixels during the transfer phase which is responsible for the very high transfer rate. The second charge sheet positioned at the wafer surface enables a very low resistance ohmic contact to the top metal contact. It is this ohmic contact that enables the HFET, which is the fundamental field effect device in the technology. The inversion channel is comprised of multiple quantum wells and these quantum wells may absorb incident radiation in the MWIR (mid-wave infrared) and LWIR (long-wave infrared) regions. The CCD may also image signals in the UV, the visible and the near IR regions of the spectrum by conventional band gap absorption.
In the CCD device of the parent application Ser. No. 09/556,285, a refractory emitter or gate metal contact is used for the transfer portion of the pixel. Also, dielectrics are used above the gate to form ¼ wavelength pairs for the imaging portion of the pixel. These dielectrics, when taken together with an epitaxially grown mirror below the active device structure constitute a resonant cavity at the wavelength of interest. In the described embodiments, ion implants are used for several purposes. An N type implant is used to form source and drain regions to the inversion channel, and it is also used to shift the threshold voltage of the inversion channel interface. The epitaxial structure is grown as a normally off (enhancement) device and then the N type implant is used to create regions of normally on (depletion) devices and it is these regions where the charge packets are stored. Oxygen implants may also be used to create high resistance regions below the implants. The technology utilizes the oxidation of AlAs and other layers with large aluminum percentages to achieve passivation, isolation and dielectric mirrors below the structure. The basic structure of the pixel and the output amplifiers which are employed in the CCD may also be used to design an active pixel sensor. In such a design, each pixel is interfaced to an output amplifier and a row or a column is output in parallel.
While the devices disclosed in parent application U.S. Ser. No. 09/556,285 represent major advances over the prior art and provide practical solutions to the problems of the prior art, the disclosed devices still suffer from certain limitations. For example, with the provided structure, it can take one millisecond or so for electrons to remove themselves from the quantum wells in response to received light, and so the speed of the imaging process is limited by that timing. In addition, because the read-out signal generated by the disclosed device for a given period is the amount of charge remaining in the quantum wells, the read-out signal is large when the well is relatively full; i.e., when the light was weak. Thus, a weak signal is possibly undesirably subjected to a relatively large amount of noise.